Digital photography has been growing fast for several years and the general public now has access to efficient and reasonably priced digital cameras. Therefore people are seeking to be able to produce photographic prints from a simple computer and its printer, with the best possible quality.
Many printers, especially those linked to personal office automation, use the inkjet printing technique. There are two major families of inkjet printing techniques: continuous jet and drop-on-demand.
Continuous jet is the simpler system. Pressurized ink (3.105 Pa) is forced to go through one or more nozzles so that the ink is transformed into a flow of droplets. In order to obtain the most regular possible sizes and spaces between drops, regular pressure pulses are sent using for example a piezoelectric crystal in contact with the ink with high frequency (up to 1 MHz) alternating current (AC) power supply. So that a message can be printed using a single nozzle, every drop must be individually controlled and directed. Electrostatic energy is used for this: an electrode is placed around the inkjet at the place where drops form. The jet is charged by induction and every drop henceforth carries a charge whose value depends on the applied voltage. The drops then pass between two deflecting plates charged with the opposite sign and then follow a given direction, the amplitude of the movement being proportional to the charge carried by each of them. To prevent other drops from reaching the paper, they are left uncharged: so, instead of going to the support they continue their path without being deflected and go directly into a container. The ink is then filtered and can be reused.
The other category of inkjet printer is drop-on-demand (DOD). This constitutes the basis of inkjet printers used in office automation. With this method, the pressure in the ink cartridge is not maintained constant but is applied when a character has to be formed. In one widespread system there is a row of 12 open nozzles, each of them being activated by a piezoelectric crystal. The ink contained in the head is given a pulse: the piezo element contracts with an electric voltage, which causes a decrease of volume, causing the expulsion of the drop by the nozzle. When the element resumes its initial shape, it pumps into the reservoir the ink necessary for new printings. The row of nozzles is thus used to generate a column matrix, so that no deflection of the drop is necessary. One variation of this system consists in replacing the piezoelectric crystals by small heating elements behind each nozzle. The drops are ejected following the forming of bubbles of solvent vapor. The volume increase enables the expulsion of the drop. Finally, there is a pulsed inkjet system in which the ink is solid at ambient temperature. The print head thus has to be heated so that the ink liquefies and it can print. This enables rapid drying on a wider range of products than conventional systems.
There now exist new “inkjet” printers capable of producing photographic images of excellent quality. However, they cannot supply good proofs if inferior quality printing paper is used. The choice of printing paper is fundamental for the quality of the obtained image. The printing paper must combine the following properties: high-quality printed image, rapid drying during printing, good image colorfastness over time, and smooth and glossy appearance.
In general, the printing paper comprises a support coated with one or more layers according to the properties required. Two main technologies have been developed. On the one hand, there is a non-porous printing paper, usually comprising layers of polymers, such as gelatin. This paper enables images that are glossy and ozone stable to be obtained because, once the polymer layer is dry, permeability to ozone is low. However, these papers, not being porous, have to swell to absorb the ink. This swelling slows ink absorption so that the ink can easily run just after printing.
Another paper has been developed to obtain a paper with rapid drying in order to increase printing output rates. This is a paper comprising a porous ink-receiving layer comprising fillers. This porous paper absorbs the ink rapidly thanks to the pores existing between the fillers. The paper can also comprise a primary tie layer as well as a protective layer or surface layer to ensure the material's glossiness. The protective layer is designed to ensure protection against fingerprints and the pressure marks of the printer feed rollers.
The porous ink-receiving layer comprises fillers used as receiving agent and a polymer binder. It enables the liquid part of the water-based ink composition to be absorbed after image creation. Elimination of the liquid reduces the risk of ink migration at the surface. The ink-receiving layer also prevents any dye loss in the fibers of the paper base to obtain good color saturation while preventing excess ink that would encourage the increase in size of the printing dots and reduce the image quality.
The purpose of the receiving agent is to fix the dyes in the printing paper. The best-known inorganic receivers are colloidal silica or boehmite. For example, European Patent Applications EP-A-976,571 and EP-A-1,162,076 describe materials for inkjet printing in which the ink-receiving layer contains as inorganic receivers Ludox™ CL (colloidal silica) marketed by Grace Corporation or Dispal™ (colloidal boehmite) marketed by Sasol. However, printing paper comprising a porous ink-receiving layer can have poor ozone stability in time, which is demonstrated by a loss of color density. In particular this is due to the fact that the fillers are easily accessible to ozone and the surface of these fillers could catalyze the ozone degradation of the ink dyes.
To meet new market needs in terms of color stability to ozone, it is necessary to propose a method for treating an inkjet recording element enabling the improvement of the printed image quality, and more particularly of the color stability to ozone of the printed image, and of the glossiness.